📄 make-06.html
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第六章：</span><span lang="EN-US" style="font-size: 18.0pt">Makefile</span><span style="font-size: 18.0pt; font-family: 楷体_GB2312">中的变量</span></a></h1>
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在</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: black">Makefile</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">中，变量是一个名字（像是</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: black">C</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">语言中的宏），代表一个文本字符串（变量的值）。在</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: black">Makefile</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">的目标、依赖、命令中引用变量的地方，变量会被它的值所取代（与</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: black">C</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">语言中宏引用的方式相同，因此其他版本的</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: black">make</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">也把变量称之为“宏”）。在</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: black">Makefile</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">中变量有以下几个特征：</span></p>
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1.<span style="font:7.0pt "Times New Roman""> </span>Makefile</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">中变量和函数的展开（除规则命令行中的变量和函数以外），是在</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: black">make</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">读取</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: black">makefile</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">文件时进行的，这里的变量包括了使用“</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: black">=</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">”定义和使用指示符“</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: black">define</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">”定义的。</span></p>
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2.<span style="font:7.0pt "Times New Roman""> </span></span>
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变量可以用来代表一个文件名列表、编译选项列表、程序运行的选项参数列表、搜索源文件的目录列表、编译输出的目录列表和所有我们能够想到的事物。</span></p>
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3.<span style="font:7.0pt "Times New Roman""> </span></span>
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变量名是不包括“</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: black">:</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">”、“</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: black">#</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">”、“</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: black">=</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">”、前置空白和尾空白的任何字符串。需要注意的是，尽管在</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: black">GNU
make</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">中没有对变量的命名有其它的限制，但定义一个包含除字母、数字和下划线以外的变量的做法也是不可取的，因为除字母、数字和下划线以外的其它字符可能会在</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: black">make</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">的后续版本中被赋予特殊含义，并且这样命名的变量对于一些</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: black">shell</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">来说是不能被作为环境变量来使用的。</span></p>
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4.<span style="font:7.0pt "Times New Roman""> </span></span>
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变量名是大小写敏感的。变量“</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: black">foo</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">”、“</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: black">Foo</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">”和“</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: black">FOO</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">”指的是三个不同的变量。</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: black">Makefile</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">传统做法是变量名是全采用大写的方式。推荐的做法是在对于内部定义定义的一般变量（例如：目标文件列表</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: black">objects</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">）使用小写方式，而对于一些参数列表（例如：编译选项</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: black">CFLAGS</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">）采用大写方式，但这并不是要求的。但需要强调一点：对于一个工程，所有</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: black">Makefile</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">中的变量命名应保持一种风格，否则会显得你是一个蹩脚的程序员（就像代码的变量命名风格一样）。</span></p>
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5.<span style="font:7.0pt "Times New Roman""> </span></span>
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另外有一些变量名只包含了一个或者很少的几个特殊的字符（符号）。称它们为自动化变量。像“</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: black">$<</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">”、“</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: black">$@</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">”、“</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: black">$?</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">”、“</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: black">$*</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">”等。</span></p>
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<h2><a name="_gnu_make_6.1"><span style="font-size: 14pt" lang="en-us"><b>6</b></span><b><span lang="EN-US" style="font-size: 14.0pt">.1<span style="font-style: normal; font-variant: normal; font-weight: normal; font-size: 7.0pt; font-family: Times New Roman">
</span></span></b><span style="font-size: 14.0pt; font-family: 楷体_GB2312">变量的引用</span></a></h2>
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当我们定义了一个变量之后，就可以在</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: black">Makefile</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">的很多地方使用这个变量。变量的引用方式是：“</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: black">$</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">（</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: black">VARIABLE_NAME</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">）”或者“</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: black">${
VARIABLE_NAME }</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">”来引用一个变量的定义。例如：“</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: black">$(foo)
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”或者“</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: black">${foo}</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">”就是取变量“</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: black">foo</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">”的值。美元符号“</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: black">$</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">”在</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: black">Makefile</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">中有特殊的含义，所有在命令或者文件名中使用“</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: black">$</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">”时需要用两个美元符号“</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: black">$$</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">”来表示。对一个变量的引用可以在</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: black">Makefile</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">的任何上下文中，目标、依赖、命令、绝大多数指示符和新变量的赋值中。这里有一个例子，其中变量保存了所有</span><span lang="EN-US" style="font-size: 12.0pt; line-height: 150%; font-family: Arial; color: black">.o</span><span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">文件的列表：</span></p>
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<span lang="EN-US" style="font-size: 12.0pt; font-family: Arial; color: black"> </span></p>
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<span lang="EN-US" style="font-size: 12.0pt; font-family: Arial">objects =
program.o foo.o utils.o</span></i></b></p>
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<span lang="EN-US" style="font-size: 12.0pt; font-family: Arial">program :
$(objects)</span></i></b></p>
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<span lang="EN-US" style="font-size: 12.0pt; font-family: Arial">cc -o program
$(objects)</span></i></b></p>
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<span lang="EN-US" style="font-size: 12.0pt; font-family: Arial"> </span></i></b></p>
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<span lang="EN-US" style="font-size: 12.0pt; font-family: Arial">$(objects) :
defs.h</span></i></b></p>
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<span lang="EN-US" style="font-size: 12.0pt; font-family: Arial"> </span></i></b></p>
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变量引用的展开过程是严格的文本替换过程，就是说变量值的字符串被精确的展开在变量被引用的地方。因此规则：</span></p>
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<span lang="EN-US" style="font-size: 12.0pt; font-family: Arial; color: black"> </span></p>
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<span lang="EN-US" style="font-size: 12.0pt; font-family: Arial">foo = c</span></i></b></p>
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<span lang="EN-US" style="font-size: 12.0pt; font-family: Arial">prog.o :
prog.$(foo)</span></i></b></p>
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<span lang="EN-US" style="font-size: 12.0pt; font-family: Arial">$(foo) $(foo)
-$(foo) prog.$(foo)</span></i></b></p>
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<span lang="EN-US" style="font-size: 12.0pt; font-family: Arial"> </span></p>
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<span style="font-size: 12.0pt; line-height: 150%; font-family: 楷体_GB2312; color: black">
被展开后就是：</span></p>
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<span lang="EN-US" style="font-size: 12.0pt; font-family: Arial; color: black"> </span></p>
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<span lang="EN-US" style="font-size: 12.0pt; font-family: Arial">prog.c : prog.c</span></i></b></p>
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<span lang="EN-US" style="font-size: 12.0pt; font-family: Arial">cc -c prog.c</span></i></b></p>
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</span></p>
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